1. Field of the Invention
This invention relates to an apparatus for detecting the deterioration of a three-way catalytic converter for an internal combustion engine, and especially relates to an apparatus for detecting the deterioration of the converter enabled to improve a detecting accuracy.
2. Description of the Related Art
In order to purify the exhaust gas exhausted from an internal combustion engine for an automobile, it is general practice to apply a three-way catalytic converter which oxidizes unburned combustibles (HC, CO) and simultaneously deoxidizes nitrogen oxides (NO.sub.x). It is necessary to control and air-fuel ratio (A/F) of the mixture supplied to the engine at the stoichiometric air-fuel ratio to maintain the oxidizing power and the deoxidizing power of the three-way catalytic converter. Therefore, a fuel injection control system for the engine uses an O.sub.2 sensor (an oxygen density sensor) See FIG. 1! which determines whether or not the air-fuel ratio of the mixture is larger than the stoichiometric air-fuel ratio, and feeds back the output of the O.sub.2 sensor in order to control the quantity of injected fuel.
In this air-fuel ratio feedback control system, the O.sub.2 sensor is mounted close to a combustion chamber, that is, at the upstream side of the converter, but the output of the O.sub.2 sensor may fluctuate due to the heat radiated from the engine. In order to compensate for this fluctuation, a double O.sub.2 sensor control system which has one more O.sub.2 sensor mounted at the downstream side of the converter has been proposed. The output of the downstream O.sub.2 sensor changes more moderately than that of the upstream O.sub.2 sensor and the downstream O.sub.2 sensor can detect an air-fuel ratio of the whole mixture because the exhaust gas at the downstream side of the converter is sufficiently mixed and its oxygen density is almost at equilibrium condition. The double O.sub.2 sensor control system has not only a main air fuel ratio feedback control subsystem using the output of the upstream O.sub.2 sensor, but also an auxiliary air-fuel ratio feedback control subsystem using the output of the downstream O.sub.2 sensor. This aims at compensating for the output of the downstream O.sub.2 sensor and an improvement of the control accuracy of the air-fuel ratio control system by correcting the air-fuel ratio correction factor used in the main air fuel ratio feedback control subsystem in accordance with the output of the downstream O.sub.2 sensor.
Sufficient purification of the exhaust gas, however, cannot be accomplished even by the above-mentioned accurate air-fuel ratio control when the converter has been deteriorated by the heat radiated from the exhaust gas or lead contained in the exhaust gas. Therefore, various kind of apparatuses for detecting the deterioration of the converter have been proposed. One type of these apparatuses detects the deterioration by detecting a decline of the O.sub.2 storage ability of the converter (the ability to store the excessive oxygen in the converter when the exhaust gas is lean and release the stored oxygen for purifying the exhaust gas when the exhaust gas is rich) by using the downstream O.sub.2 sensor. That is, as the deterioration of the converter causes the fall of the O.sub.2 storage ability of the converter, this type apparatus detects the deterioration of the converter by detecting the fall of the O.sub.2 storage ability shown by a trajectory length (that is, the length of the response curve of the sensor) and/or an inverting frequency of the output of the downstream O.sub.2 sensor. For example, the apparatus disclosed in the Unexamined Patent Publication No. 5-98948 detects the deterioration of the converter by measuring lengths of the response curves the outputs of the upstream O.sub.2 sensor and the downstream O.sub.2 sensor, and calculating the ratio between these two lengths.
Recently, an engine control system which controls an air-fuel ratio so that a purification power of the converter can be always maintained at a high level has been developed. That is, the O.sub.2 storage ability means a power for purifying the exhaust gas by storing excessive oxygen when the exhaust gas is lean, and by releasing the stored oxygen when the exhaust gas is rich, but this power is limited. Therefore, it is necessary to keep an amount of oxygen stored in the converter at a fixed amount (for example, a half of the maximum amount) in order to provide for the air-fuel ratio fluctuating from the rich state to the lean state and vice versa. If the amount of the stored oxygen is kept at the fixed amount, it is possible to store oxygen in the converter or release it from the converter, and to maintain the oxidizing power and deoxidizing power of the converter at a high level.
In the above-mentioned engine control system, an air-fuel ratio sensor which can linearly detect the air-fuel ratio (A/F sensor) See FIG. 2! is used as the upstream sensor, and a proportional plus integral feedback control scheme (PI control scheme) is applied. That is, an injected fuel amount is calculated using a following equation. EQU Revised fuel amount=K.sub.p .times.(Present fuel error)+K.sub.s .times..upsilon.(Previous fuel error)
Where,
Fuel error=Fuel amount actually burned in a cylinder-Fuel amount required to keep a stoichiometric air-fuel ratio PA1 Fuel amount actually burned in a cylinder=Actual air flow/Actual air-fuel ratio PA1 K.sub.p =Proportional gain PA1 K.sub.s =Integral gain PA1 1. In the case of the minimum fluctuation of the output of the upstream sensor VAF PA1 2. In the case of the small fluctuation PA1 3. In the case of the medium LVAF PA1 4. In the case of the large LVAF PA1 5. In the case of the maximum LVAF
As shown in the above equation, the proportional term functions so as to control the actual air-fuel ratio at the stoichiometric air-fuel ratio similarly when the actual air-fuel ratio is controlled in accordance with the output of the O.sub.2 sensor, and the integral term functions so as to cancel the offset. That is, the amount of the stored oxygen can be maintained at a fixed amount by this integral term. For example, if the lean exhaust gas is supplied due to a sudden speed up, the oxygen in the exhaust gas is stored in the converter to compensate for the lean state of the exhaust gas.
For the above-mentioned O.sub.2 storage amount control system, an O.sub.2 sensor mounted at the downstream side of the converter can be also applied in order to compensate for the fluctuation of the upstream A/F sensor. Therefore, the deterioration of the converter can be detected by detecting the fall of the O.sub.2 storage effect using the downstream O.sub.2 sensor.
By the way, the output of the downstream O.sub.2 sensor is influenced by the fluctuation of the air-fuel ratio of the exhaust gas supplied to the converter even when the converter is not deteriorated. That is because the output of the downstream O.sub.2 sensor is influenced by the fluctuation of the air-fuel ratio of the exhaust gas exhausted from the converter which depends upon that of the exhaust gas supplied to the converter. Therefore, a misjudgement of the deterioration of the converter may be caused due to the fluctuation of the air-fuel ratio of the exhaust gas supplied to the converter when the deterioration of the converter is detected in accordance with the output of the downstream sensor. The reason why the misjudgement is caused will be explained referring to the accompanying drawings.
FIG. 3 shows various outputs of the downstream O.sub.2 sensor, which correspond to minimum, small, medium, large, and maximum fluctuations of the air-fuel ratio of the exhaust gas supplied to the converter (that is, the output of the upstream A/F sensor VAF), not only when the converter is normal, but also when it is deteriorated.
In this case, the length of the response curve of the upstream A/F sensor LVAF becomes minimum. The fluctuation of the output of the downstream sensor VOS and the length of its response curve LVOS also become minimum, not only when the converter is normal, but also when it is deteriorated. Therefore, the ratio of these lengths, that is, the ratio of LVAF to LVOS, becomes about 1.0 whether or not the converter is normal. In this case, the condition of the converter cannot be detected by the ratio of the these lengths.
The length ratio becomes about 0.5 when the converter is normal, because the length of the response curve of the downstream sensor LVOS is kept minimum, though LVAF increases from minimum to short.
But the length ratio becomes 2.0 when the converter is deteriorated, because LVOS becomes medium as the O.sub.2 storage ability does not function. In this case, the condition of the converter can be detected by the ratio of the lengths.
The length ratio becomes about 0.2 when the converter is normal, because LVOS is kept short due to the O.sub.2 storage ability, though LVAF increases to medium.
However, the length ratio becomes 1.5 when the converter is deteriorated, because LVOS becomes long due to the Z shape-output characteristics of the O.sub.2 sensor.
In this case, the condition of the converter can be detected by the ratio of the lengths.
The length ratio becomes about 0.4 when the converter is normal, because LVOS is kept medium due to the O.sub.2 storage effect.
However, the length ratio becomes 1.0 when the converter is deteriorated, because LVOS becomes long due to the Z shape-output characteristics of the O.sub.2 sensor.
In this case, the condition of the converter can be detected by the ratio of the lengths.
The length ratio becomes about 0.6 when the converter is normal, because LVOS becomes large due to the Z shape-output characteristics of the O.sub.2 sensor whether or not the converter is normal. In this case, the condition of the converter cannot be detected by the ratio of the lengths.
Therefore, the detection of the deterioration of the converter must be inhibited when LVAF, that is, the length of the response curve of the output of the upstream A/F sensor is minimum or maximum if the deterioration is detected based on the lengths or the length ratio. Therefore, when the deterioration is detected by monitoring the outputs of the both sensors and calculating their lengths for a predetermined fixed monitoring period, it is possible to prevent the misjudgement by detecting the deterioration only when LVAF is between a predetermined fixed upper limit and a predetermined fixed lower limit.
Though this method is effective when VAF has a constant amplitude as shown FIG. 3, this method may cause misjudgement under a specific worst-case condition. Namely, when the air-fuel ratio feedback control system using the output of the A/F sensor is applied, the amplitude of the output of the A/F sensor may vary in accordance with various running conditions, because the air-fuel ratio is controlled so that the correction factor is increased proportional to the difference between the output of the sensor and the target voltage (corresponding to the stoichiometric air-fuel ratio). Therefore, the air-fuel ratio is kept constant while the automobile is running at constant speed, but it fluctuates when the speed is transiently changed.
For example, FIG. 4 shows a case where the air-fuel ratio is suddenly fluctuated for a monitoring period. The monitoring period is divided into two parts, that is, one is a period where the amplitude of VAF becomes maximum, and another is a period where that is kept at minimum as shown FIG. 4(A). In this case, VOS shows the same shape whether or not the converter is normal (See FIG. 4(B)), because VOS is kept minimum when VAF is minimum and VOS becomes large when VAF is maximum. The detection of the deterioration, however, is carried out because the length of the response curve of the A/F sensor LVAF for the monitoring period becomes longer than the predetermined lower limit. A misjudgement that the converter is normal after it has been actually deteriorated may be caused because LVOS for the monitoring period is kept short.
FIG. 5 shows a case where the running condition changes from the condition with large fluctuations of the air-fuel ratio to a normal condition for a monitoring period. The monitoring period is also divided into two parts, that is, one is a period where the amplitude of VAF becomes maximum, and another is a period where that is kept at minimum as shown FIG. 5(A). In this case, VOS also shows the same shape whether or not the converter is normal (See FIG. 5(B)). The detection of the deterioration, however, is carried out because the length of the response curve of the A/F sensor LVAF is averaged for the monitoring period and becomes shorter than the predetermined upper limit. The misjudgement that the converter is deteriorated even when it is actually kept normal, may be caused because the LVOS for monitoring period becomes long.
Furthermore, the accuracy for detecting the deterioration may become worse if the balance between the power for storing oxygen and the power for releasing oxygen is lost, because the amplitude of the downstream O.sub.2 sensor is influenced by this balance even when the air-fuel ratio of the exhaust gas supplied to the converter is kept constant.
FIG. 6 is a graph to explain the above-mentioned problem, where (A) shows the output of the upstream A/F sensor and (B) shows the output of the downstream O.sub.2 sensor. In this graph, the output of the O.sub.2 sensor is kept at the rich state before time t.sub.3, and turns from the rich state to the lean state at time t.sub.3.
When the air-fuel ratio of the exhaust gas supplied to the converter deviates largely to the lean state at time t.sub.1, the length of the response curve of the output of the O.sub.2 sensor becomes long because the output of the O.sub.2 sensor also deviates largely to the lean state from the rich state.
Conversely, when the air-fuel ratio of the exhaust gas supplied to the converter deviates largely to the rich state at time t.sub.2, the length of the response curve of the O.sub.2 sensor is not largely influenced by this deviation because the output of the O.sub.2 sensor becomes saturated.
Similarly, when the air-fuel ratio of the exhaust gas supplied to the converter deviates largely to the lean state at time t.sub.4, the length of the response curve of the O.sub.2 sensor is not largely influenced by this deviation because the output of the O.sub.2 sensor becomes saturated.
Conversely, when the air-fuel ratio of the exhaust gas supplied to the converter deviates largely to the rich state at time t.sub.5, the length of the response curve of the O.sub.2 sensor becomes long because the output of the O.sub.2 sensor also deviates largely to the rich state from the lean state.
That is, the accuracy for detecting the deterioration of the converter may become worst because the deviation of the O.sub.2 sensor is affected by the fluctuation of the air-fuel ratio of the exhaust gas exhausted from the converter, even when the fluctuation of the exhaust gas supplied to the converter is kept constant.
Note, the same problem may be caused if the downstream sensor is an A/F sensor.